Theory, experiments, and field measurements show that metalworking fluid mists can evaporate into passing air after they have been collected by filters. These evaporative losses will lead to negative biases in concentration measurements. The extent of these losses has not been delineated fully. Although the degree of bias depends on many factors such as fluid composition, ambient temperature, and dilution of mist prior to sampling, evaporation is likely to occur whenever the vapor phase of the sampled air is not saturated with the compounds accumulated on the filter. The proposed research will evaluate evaporative losses during mist sampling systematically and will lead to methods to correct for biases in concentration measurements. As a result of the project, worker exposures to metalworking fluid aerosols will be assessed more accurately. The proposed work has three parts: (a) study evaporation from a theoretical standpoint. Using existing numerical models and new ones to be developed, evaluated the importance of the factors affecting evaporation of sampled metalworking fluids from filters; (b) study evaporation in laboratory experiments. Based on results from the modeling, select conditions for tests in the laboratory to verify the importance of these conditions upon evaporative losses. These experiments will validate the numerical modeling; study evaporation in the field. In large and small facilities, determine evaporative losses over a wide range of conditions. With the results, we will determine how the models can be used to predict accurately the amount of evaporation occurring in real facilities. If evaporative losses in the field can be predicted, researchers will be able to reevaluate historical mist concentration data to determine if evaporation has been a problem in the past. Reevaluated data may be able to be used more effectively in epidemiological studies on the health effects of metalworking fluid mists.